In general, polycarbonate (PC), polybutylene terephthalate (PBT), polyethylene terephthalate (PET), blends thereof and thermoplastic resins obtained by blending one or a plurality of these resins with other resins cause heat decomposition when heated and molten in various molding processes and are reduced in a molecular weight, which results in a reduction in the mechanical properties. The present invention relates to a resin additive for preventing heat decomposition in such case and a heat stabilizing method of a thermoplastic resin using the above resin additive.
Resin materials, particularly the respective resins of PC, PBT and PET are heavily reduced in a molecular weight in heating and melting and therefore have notably been reduced in a molecular weight by virtue of heat given in the respective molding processes and a shearing stress in kneading. Accordingly, it has been difficult to recycle sprues and runners produced in molding as well as recycling of the products. An object of the present invention is to provide a novel resin additive which inhibits the preceding reduction in a molecular weight of resin materials in heating and melting and which can turn them into recyclable materials and a heat stabilizing method of a thermoplastic resin using the above resin additive.
The object of the present invention described above can be achieved by:
(1) a resin additive prepared by dehydrating tannin by heating up to 70 to 230xc2x0 C. to subject it to polycondensation, and
(2) a heat stabilizing method of a thermoplastic resin characterized by adding 50 to 1000 ppm of the above resin additive to the thermoplastic resin. The present invention shall be explained below in further details.
Intensive researches of an influence exerted on materials by polyhydric phenol continued by the present inventors have resulted in finding that a thermoplastic resin can be inhibited from being reduced in a molecular weight in heating and melting by adding a trace amount of tannin dehydrated by heating and thus, they have completed the present invention. The tannin used in the present invention is a general term of complicated aromatic compounds having a lot of phenolic hydroxyl groups which are widely distributed in the plant kingdom, and to roughly divide, tannin is divided into two kinds of a hydrolyzed type and a condensed type. Either of them is a natural compound, and therefore a lot of the compounds having different structures are present. Either tannin may be used in the present invention. Polyhydric phenol compounds having a dye-fixing effect and a tanning effect of leather are called xe2x80x9csynthetic tanninxe2x80x9d and xe2x80x9ccintanxe2x80x9d, and among the synthetic tannins, the compounds which are effectively used can be used as well in the present invention.
Also, tannic acid is called tannin as well and shall not specifically be distinguished in the present invention.
China tannin which is typical hydrolyzed type tannin is shown by Formula (1). To further describe the China tannin, it has been apparent that the China tannin has a structure in which 10 gallic acid groups are disposed in the circumference of a glucose residue and two gallic acid groups are bonded in a vertical direction. However, the center of the compound is not necessarily restricted to glucose and is the compound of a cellulose type in a certain case. Thus, tannins are compounds contained widely in plants in the natural world, and therefore it can readily be presumed that they are partially different in a chemical structure.
Tannins including decomposition products obtained by alkaline decomposition or hydrolytic decomposition of tannin are used as well in the present invention and, for example, didepside of gallic acid shown by Formula (2) which is obtained by hydrolysis of tannin can be used as well. At present, tannin is used for daily needs such as an ink, medical uses such as a hemostatic agent and industrial uses such as a tanning agent of leather and a mordant in dyeing, and in recent years, it is used as a food additive. Further, tannins including catechin are used as well in the present invention. Catechin is a polyhydroxy derivative of 3-hydroxyflavane which is a polyhydric phenol compound and contained widely in plants in the natural world. It is said that catechin includes heterogeneous types having various molecular structures, and any ones are natural compounds, so that a lot of catechins having different structures are present. Catechin used in the present invention shall not specifically be restricted and may be any one as long as it is catechin which can effectively be used for the object of the present invention. Also, catechin is called shibu. At present, tannin is used for medical uses such as a carcinostatic agent and industrial uses such as a color fixing agent and a mordant for nylon.
Tannin preferably used in the present invention includes China tannin and 3,5,7,3xe2x80x2,4xe2x80x2-pentahydroxyflavane (catechin in terms of a narrow sense). Catechin (3), quebrotannin (4) and turkey tannin (5) are shown in Formula (3), Formula (4) and Formula (5) as various tannins having different chemical structures. 
The tannin of the present invention subjected to polycondensation can be produced by heating and dehydrating tannin at a temperature of up to 70 to 230xc2x0 C., preferably 90 to 160xc2x0 C. In the present invention, a polycondensation product of one kind of tannin may be used or a polycondensation product of a mixture of several kinds of tannin may be used. In this case, it is important that tannin is dehydrated to some extent, and the whole thereof may not necessarily be subjected to polycondensation.
Conditions in heating and dehydrating tannin to subject it to polycondensation shall not specifically be restricted, and it is usually heated at 70xc2x0 C. for about 8 hours, 230xc2x0 C. for about 2 minutes and 90 to 160xc2x0 C. for about 2 to 3 hours to dehydrate it. In the case of China tannin, the product which is dehydrated by not much more than 8 to 11% is preferably used.
Next, a resin to which the resin additive of the present invention is intended to be added shall not specifically be restricted, but polycarbonate resins (PC) or thermoplastic polyesters, particularly polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) are suitable. Further, preferably used are blends of them and blend resins of one or a combination of a plural thereof and other resins, for example, blend resins such as PC/ABS, PBT/ABS, PC/PBT, PC/PET, PC/polystyrene and PC/PBT/polystyrene.
Polycondensation tannin is added in an amount of 50 to 1000 ppm based on the thermoplastic resin. The addition amount of less than 50 ppm is too small and does not provide so much effect. On the other hand, the addition amount exceeding 1000 ppm rather deteriorates the mechanical properties of the resin and therefore is not preferred.
Powdery polycondensation tannin may be added directly to the resin or polycondensation tannin may be dissolved in a solvent such as THF and ethanol, and this may be added to the resin. The polycondensation tannin of the present invention has the characteristic that the heat stabilizing effect of the resin is obtained by adding a very small amount of 50 to 1000 ppm thereof. Further, polycondensation tannin has the advantages that an OH group contained in tannin is reduced to some extent by polycondensation, and therefore when this is added to the thermoplastic resin, it can prevent the OH groups which are present in excess from being decomposed in heating and melting the resin and foaming in the resin to form bubbles and that it can further thermally stabilize the thermoplastic resin.
The thermoplastic resin thus obtained to which polycondensation tannin is added is inhibited from a reduction in a molecular weight in heating and melting and makes it possible to recycle the molded article.
As shown in the following examples and comparative examples, the addition of tannin subjected to heating, dehydrating and polycondensation which is the resin additive of the present invention inhibits the respective PC or polyester base resins from being reduced in a molecular weight in heating and melting. Accordingly, heat stabilization of a thermoplastic resin can be achieved by using the resin additive of the present invention.